White dwarf boosts light of stellar companion

Rare gravitational lens in binary system may reveal properties of exotic stars

MAGNIFYING STAR A white dwarf, the core of a dead star, contorts the light of the sunlike star it orbits, as seen in an artist’s illustration. The white dwarf’s gravity amplifies the light from its companion each time the white dwarf passes between the other star and Earth.

Astronomers have discovered a unique pair of stars consisting of a white dwarf, the compact core of a dead star, and the sunlike star it orbits. When the white dwarf passes in front of its companion, as seen from Earth, the white dwarf’s gravity magnifies the other star’s light. The pair represents the first clear sign of a gravitational lens in a binary star.

The white dwarf — with a mass about 60 percent of the sun’s and a volume not much bigger than Earth’s — orbits a sun doppelgänger every 88 days, astronomers report in the April 18 Science. Ethan Kruse and Eric Agol of the University of Washington in Seattle discovered the binary, lurking 2,600 light-years away in the constellation Lyra, while mining data collected by the Kepler space telescope. Agol and another researcher had predicted that Kepler might find about a dozen self-lensing binaries; this is the first to turn up.

Self-lensing binaries provide a rare opportunity to directly measure masses of stars that otherwise could not be measured. Doing so might help scientists unravel the exotic physics of white dwarfs and the evolution of binaries, which make up nearly 40 percent of the sunlike stars in the galaxy.